What will never be included and why or what cannot be done with
this concept.

oracles single tasking possibility

Oracle version 7.0 on AIX 3 uses the OS-supported locks on
the shared memory segments and allows the application designer
to link an application in a so called single tasking way.
Instead of starting one client process per application process
both the database part and the application part is run in the
same process. In later versions of oracle this is no longer
supported.

This would require a total redesign of the Postgres access model and that effort can
not justify the performance gained.